Endogenous and exogenous components of circadian rhythms when living on a 21-hour day.

The effects of living on a 21-h sleep-wakeful cycle upon the rhythms of urinary electrolyte excretion and deep body temperature were measured in a single female subject in an isolation unit. The effects of this 21-h routine upon the endogenous components of these rhythms were assessed by studying the subject, after 4 days on this routine, on a 'constant routine' in which exogenous rhythmic influences were removed, and the absence of any timepiece. The results confirm conclusions from previous studies which indicate that during a 21-h routine both endogenous and exogenous components are simultaneously present, though the relative size of these components depended on the variable under consideration. The endogenous component appeared to free-run with a period longer than 24 h. In the second experiment (when all timepieces were removed at the end of the time on a 21-h routine) the sleep-wakeful cycle continued to oscillate for several cycles with a period near 21 h. This suggests that activity was controlled by an oscillator which could be entrained to a period of 21 h.